Measure encountered by Her Majesty (5)
I believe the answer is:
meter
'measure' is the definition.
(meter is a kind of measure)
'encountered by her majesty' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter'**).
'by' is a charade indicator (letters next to each other).
'her majesty' becomes 'er' (Elizabeth Regina).
'met'+'er'='METER'
(Other definitions for meter that I've seen before include "Measuring device as for electricity usage (5)" , "Device for measuring and recording" , "Gauge" , "Measuring device as for electricity consumption" , "Device for recording, say, usage (5)" .)